Neisseria meningitidis is a Gram-negative encapsulated bacterium which colonises the upper respiratory tract of approximately 10% of human population. Conjugate vaccines are available against serogroups A, C, W135 and Y, but the only vaccine which is available for protecting against serogroup B in a two-dose regimen is the BEXSERO™ product which was approved in 2013.
One of the protective immunogens in BEXSERO™ is fHbp, which has also been known as protein ‘741’ (SEQ ID NO: 2536 in ref. 1; SEQ ID 1 herein), ‘NMB1870’, ‘GNA1870’ [2-4, ‘P2086’, ‘LP2086’ or ‘ORF2086’ [5-7]. The 3D structure of this protein is known [8,9], and the protein has two β-barrels connected by a short linker. Many publications have reported on the protective efficacy of this protein in meningococcal vaccines e.g. see references 10-14. The fHbp lipoprotein is expressed in various strains across all serogroups. fHbp sequences have been grouped into three variants [2] (referred to herein as v1, v2 and v3), and it has been found in general that serum raised against a given variant is bactericidal against strains which express that variant, but is not active against strains which express one of the other two variants i.e. there is intra-variant cross-protection, but not inter-variant cross-protection (except for some v2 and v3 cross-reactivity).
To increase inter-family cross-reactivity the fHbp sequence has been engineered to contain specificities for all three variants [15]. Protein engineering has also been used to remove fHbp's interaction with siderophores [16] and with factor H [17-25]. Disruption of the interaction with fH has been reported for all three variants and is postulated to provide a superior vaccine immunogen [22,26]. For v2 polypeptides, however, references 23 and 24 report an inherent instability which is also seen in mutants with disrupted fH-binding. The instability appears to arise from the N-terminal β-barrel domain, and reference 23 warns that any substitutions in this barrel might promote instability.
It is an object of the invention to provide further fHbp v2 and v3 mutants, but having enhanced stability.